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On Page 48 of the book it mentions that based on the Guttenberg-Richter scale, size 4 earthquakes are 10 times stronger than size 3 quakes.

The magnitude of earthquakes, as measured by the seismic moment, (a very low frequency component of the energy) goes up by a factor of 101.5= 31.6 for every step on the scale. Thus, a magnitude 4 earthquake is actually almost 32 times more energetic as a magnitude 3.